1. Field of the Application
The present application relates, in general, to electronic circuits which utilize amplifiers.
2. Description of the Related Art
With reference to the figures, and with reference now to FIG. 1A, shown is a schematic diagram of a classic differential amplifier circuit 100. Depicted is that for the classic differential amplifier circuit 100 shown, the gain, Gdiff, of the differential amplifier 100 is equal to Vout/(vinput—1–vinput—2), or RC/2(RE+re). Illustrated is that the common mode rejection ratio (CMRR) of the classic differential amplifier circuit 100 is approximately equal to R1/(RE+re). See Horowitz and Hill, The Art of Electronics 98–99 (1989 2ed.).
Referring now to FIG. 1B, depicted is a schematic diagram of a modified version of the classic differential amplifier circuit 100 of FIG. 1A, wherein a current source 102 has been substituted for the resistor R1. Those having ordinary skill in the art will appreciate that replacing the resistor R1 with the current source 102 greatly reduces the common-mode gain relative to the differential amplifier circuit 100, and thereby greatly improves the CMRR relative to the classic differential amplifier circuit 100. Id at 100.
With reference now to FIG. 2, illustrated is a schematic diagram of an emitter-follower circuit 200, which is called an “emitter-follower” circuit because the output terminal, Vout, is the emitter, and hence follows the input (the base) less one diode drop (e.g., VE is approximately equal to VB−0.6 volt). Hence the output voltage is a replica of the input (i.e., “follows” the input), except that output voltage is 0.6 to 0.7 V less positive. For the circuit, Vin (VBE) must stay at +0.6 volt or more, or else the output will sit at ground. Those having ordinary skill in the art will appreciate that due to the fact that the emitter resistor REF is coupled to a negative supply voltage, the emitter-follower circuit 200 will permit negative voltage swings as well. However, those skilled in the art will also appreciate that care must be taken with respect to the negative voltage swing, to ensure that the reverse bias break down voltage (VBE) is not exceeded. Id at pages 65–68. In addition, those having ordinary skill in the art will also appreciate that care must also be taken with respect to the voltage across the collector and emitter so that BVces breakdown is not exceeded, and that comparatively low BVces break down voltage is a recognized problem of typical advanced high speed IC processes.
Referring now to FIG. 3, shown is a schematic diagram of transistors QC and QA connected in what is known in the art and as a “Darlington pair” 300. Those skilled in the art will appreciate that the Darlington pair 300 tends to act like a single transistor with a current gain equal to the product of the current gains of the two individual transistors. Depicted is a resistor 302 connected between the base and emitter of the second transistor QC driven by the first transistor QA, which those skilled in the art will appreciate improves the response time of the overall Darlington pair 300. Those skilled in the art will appreciate that for a Darlington pair without the resistor 302, the combination of transistors QC and QA tends to act like a rather slow transistor because QA cannot turn off QC quickly. Those skilled in the art will appreciate that this problem is usually taken care of by including the resistor 302 from base to emitter of QA, as shown in FIG. 3. The resistor 302 also prevents leakage current from QA from biasing QC into conduction; its value is chosen so that QA's leakage current (nanoamps for small-signal transistors; as much as hundreds of microamps for power transistors) produces less than a diode drop across resistor 302 and so that resistor 302 doesn't sink a large proportion of QC's base current when it has a diode drop across it. Typically resistor 302 might be a few hundred ohms in a power transistor Darlington, or a few thousand ohms for a small-signal Darlington. Id. at pages 95–98.